The present invention is directed to a system and method for demineralizing water using ion exchange resins.
A power plant normally employs two types of water demineralization systems, namely, a condensate polishing system and a make up demineralization system. The condensate polishing system further demineralizes condensate water from a steam condenser. Condensate water is already of low solids content, e.g., normally having a total dissolved solids content below 5 parts per million (ppm). Traditional condensate polishing process can produce water having a final dissolved solids content level of about 5 to about 50 parts per billion (ppb). Condensate water that has been polished is used, substantially undiluted, in a power plant operating system. By further reducing the total dissolved solids content of water, condensate polishing processes help prevent corrosion to the power plant operating system.
Ion exchange processes were initially developed for the demineralization of water containing relatively high dissolved solids contents, i.e., above about 20 ppm. Mixed bed systems, in which the bed is a mixture of anion exchange and cation exchange resins, were long ago accepted as being advantageous for polishing condensate.
A mixed bed employed in condensate polishing processes typically comprises a mixture of strong base anion (SBA) and strong acid cation (SAC) exchange resins. These mixed bed systems are generally regenerated to reuse the resins and thereby reduce their operating costs. A strong acid, e.g., sulfuric or hydrochloric acid, is used to regenerate the SAC resin and caustic is used to regenerate the SBA resin.
There are problems with the use of mixed bed systems in condensate polishing processes. One of the most significant problems is that the SBA and SAC resins cannot be totally separated. Therefore, part of the SBA resin ends up contacting either the sulfuric or hydrochloric acid used to regenerate the SAC resin and is thereby put in the (a) sulfate or bisulfate or (b) chloride form, respectively. Similarly, part of the SAC resin contacts the caustic used to regenerate the SBA resin thereby putting the SAC resin into the sodium form. When the regenerated SAC and SBA resins are returned to the mixed bed and the mixed bed is put back into service, the SBA resins that were put in the sulfate or chloride form and the SAC resins that were put in the sodium form leak (a) sulfate or chloride and (b) sodium, respectively, from the mixed bed. This leakage, which is in the low ppb range, increases the total dissolved content of the effluent water and can also increase the effluent water's corrosion potential. Both of these adverse results can contribute to corrosion of a power plant operating system.
The mixed bed systems employed as polishers typically also suffer from poor "kinetics". Kinetics is the rate or speed at which contaminates are adsorbed onto a resin, and thereby removed from the water, at a given flow rate of water. Furthermore, the mixed bed systems require frequent regenerations when an influent to the mixed bed has a high sodium level or chloride, e.g., due to a leak in a steam condenser, or a high ammonia level due to the feed of ammonium and hydrazide into the cycle. In addition, current water polishing systems yield effluent water which possesses an unacceptably high corrosion potential. Each of these dificiencies adversely impacts the utilities operating costs.
An additional problem that mixed bed systems suffer from is that the SAC resin has some level of sulfonates constantly being leached out from the SAC resin and onto the SBA resin. Because the SBA resin's ability to adsorb sulfonates is very low and because the amount of SBA resin downstream from particular SAC resin particles varies in the mixed bed system, the sulfonates eventually leak into the effluent water from the mixed bed. The leaked sulfonates, under the high temperatures of a boiler system, break down creating sulfates which are very corrosive to a power plant operating system.
A number of efforts have been made to alleviate these problems. However, none of the previously proposed solutions satisfactorily address all these problems.
With respect to the make up demineralization system, this system demineralizes water being brought into a power plant to make up for water lost during the power plant operating cycle. Water brought into the power plant normally has a total dissolved solids content of above 10 ppm. For example, this water can be potable water. Since less than about two percent of the water employed in power plant operating cycle is make up water, make up demineralization systems, because of cost considerations, do not produce water having as low a dissolved solids content as water treated by condensate polishing systems.
Make up demineralization systems consist of two sections. The first section is for primary water treatment and the second section is for polishing effluent water from the primary water treatment section. Each section can consist of one or more ion exchange resin beds or zones. The ion exchange beds employed in the primary water treatment section generally have a larger cross-sectional area and a deeper depth than the ion exchange beds employed in the water polishing section. Effluent water from the primary water treatment section of the make up demineralization system normally has a dissolved solids content of below 10 ppm. The polishing section of the make up demineralization system traditionally produces water having a final dissolved solids content level as high as 50 ppb.
Typically, sodium is the predominant mineral in the effluent water from the make up demineralization system. Due to operating cost considerations, this final dissolved solids content is no longer satisfactory. Although efforts have been made to lower the final dissolved solids content level of the treated make up water, none of the previously proposed solutions has satisfactorily solved the problem.
Accordingly, there is a need for a system and process for polishing condensate water that are capable of producing water having a purity comparable to or better than that currently obtainable but which are devoid of the problems characteristic of current polishing systems and processes. These problems include (a) unacceptably high sulfonate, chloride, and/or sulfate leakage in the effluent water, (b) poor kinetics, (c) frequent resin regeneration requirements, (d) cross contamination of resins during regeneration procedures, and (e) undesirably high corrosion potential of the effluent water. In addition, there is a need for a make up demineralization system and process for demineralizing make up water that is capable of economically yielding an effluent water having a sodium content of less than about 5 ppb and a total dissolved solids content of less than about 10 ppb.